It is widely acknowledged that swellable synthetic polymers can be obtained through the covalent crosslinking of water-soluble polymers to form three-dimensional networks, yielding, in most cases, what is known in the industry as the precursors of hydrogels [Chapter 30 in Water Soluble Polymers (Shalaby, S. W. et al., Eds.), American Chemical Society, Washington, 1991, p. 482; Chapter II in Hydrogels in Medicine and Pharmacy, Vol. I (Peppas, N. A., Ed.), CRC Press, 1986, p. 2; and Chapter 1 in Biodegradable Hydrogels for Drug Delivery (Park, K et al.), Technomic Publishing Co., 1993, pps. 1 and 2].
A concise outline dealing with water-swellable polymer networks and related systems is available through the Sigma-Aldrich web site (www.Sigma-Aldrich.com). A paraphrased excerpt of this outline is provided below.                Hydrogels are characterized by the pronounced affinity of their chemical structures for aqueous solutions in which they swell rather than dissolve. Such polymeric networks may range from being mildly absorbing, typically retaining 30 wt. % of water within their structure, to superabsorbing, where they retain many times their weight of aqueous fluids. Several synthetic strategies have been proposed to prepare absorbent polymers: (a) polyelectrolyte(s) subjected to covalent cross-linking; (b) associative polymers consisting of hydrophilic and hydrophobic components (“effective” cross-links through hydrogen bonding); and (c) physically interpenetrating polymer networks yielding absorbent polymers of high mechanical strength. Specific descriptions of major hydrogel precursors are given below:        
Absorbent PolymerMorphologyAbsorption Characteristics1. Poly(acrylic acid), potassium salt,Powder; particle size 99%Absorbs ca. 27 g/g of 1% saline  lightly crosslinked<1,000 μmsolution; rate of absorption morerapid than for corresponding Nasalt2. Poly(acrylic acid), sodium salt,Powder; particle size 99%Absorbs ca. 45 g/g of 1% saline  lightly crosslinked<1,000 μmsolution3. Poly(acrylic acid-co-acrylamide),Granules; 200–1,000 μm;Absorbs many times its weight of  potassium salt, crosslinkedpH 5.5–6.0aqueous fluids4. Poly(acrylic acid, sodium salt-graft-Granular powder; 100–850 μmAbsorbs many times its weight of  poly(ethylene oxide), crosslinkedaqueous fluids5. Poly(2-hydroxyethyl methacrylate),Crystals—  average Mv ca. 300,0006. Poly(2-hydroxypropyl methacrylate)Crystals—7. Poly(isobutylene-co-maleic acid), sodiumFiber; 24–40 μm diameterAbsorption of 0.9 wt % saline  salt, crosslinkedsolution is ca. 65 g/g; absorption ofdistilled water is ca. 300 g/g(See the Sigma-Aldrich website: www.Sigma-Aldrich.com)
The concept of using amphiphilic, copolyesters with chain molecules comprising covalently linked hydrophilic and hydrophobic segments/blocks to produce swellable polymeric materials or hydrogels through association of the hydrophobic segments and formation of non-covalent, pseudo-crosslinks in the presence of water has been disclosed by Shalaby in U.S. Pat. Nos. 5,612,052 and 5,714,159. However, these amphiphilic copolyesters were described as hydrogel-forming liquids and lack needed properties to form solid, swellable products such as those needed in producing surgical implants, coatings of surgical implants or carriers for the delivery of bioactive agents, requiring different levels of structural integrity, which cannot be provided by the hydrogel-forming liquids. Other inventors have also disclosed the use of amphiphilic copolymers comprising polyethylene glycol interlinked with polyester as carriers for the delivery of bioactive agents [U.S. Pat. No. 5,702,711]. However, all these polymers were designed to be water-soluble materials that swell and undergo dissolution and lose their mechanical integrity. The above cited prior art and contemporary needs for swellable, space-filling implants and coatings as well as drug carriers with modulated swelling profiles provided an incentive to explore the subject of the present invention.